There is worldwide activity by the automobile manufacturers to develop automatic transmissions incorporating various continuous slip torque converter clutch (CSTCC) designs. These developments are being driven by the anticipated increase in Corporate Average Fuel Economy (CAFE) requirements in the U.S.A. The CSTCC design allows increases in fuel economy to be gained with minimal mechanical modifications to the transmission.
One of the barriers to successful implementation of the continuous slip torque converter clutch design for automatic transmissions is transmission shudder. An important factor contributing to shudder is the frictional characteristics of the automatic transmission fluid (ATF). Shudder is undesirable for the durability and operability of the equipment and can result in customer complaints and increased warranty costs. As a result, many original equipment manufacturers are looking for automatic transmission fluids with frictional characteristics capable of meeting the requirements of CSTCC designs.
The torque converter is located between the engine and transmission in an automatic transmission. It functions as a engine torque multiplier and a mechanism to transmit engine power by fluid coupling. Most of the recent transmission torque converters are equipped with lock-up clutches (or centrifugal bypass clutches). Lock-up clutches are engaged at highway speeds to reduce the energy loss due to pump/turbine inefficiencies. Further improvements in fuel economy can be achieved if the lock-up clutches are engaged at lower driving speeds. However, it is not possible to dampen the power fluctuations from the engine at low driving speeds if the lock-up clutches are completely engaged. In a CSTCC, the lock-up clutch continuously slips while engaged at lower driving speeds and can be locked up (without slippage) at highway speeds. (The terminology "continuous slip torque converter clutch" is terminology that has developed in the art, but it must be kept in mind that in spite of this terminology, the continuously slipping clutches are not necessarily slipping all of the time.) The CSTCC design not only reduces the energy losses associated with complete fluid coupling, but also allows power fluctuations to be smoothed. A vehicle equipped with a CSTCC is expected to have better fuel efficiency by approximately 10% compared to that for a conventional lock-up torque converter design transmission.
Vehicles equipped with CSTCC transmissions often suffer from the undesirable phenomenon of shudder or self-excited vibration. This vibration is believed to be caused by a "stick-slip" phenomenon, in which two surfaces alternately stick together and slip over each other; two surfaces stick when the lateral force is not great enough to overcome the frictional force and they break loose when the lateral force builds up enough to overcome frictional forces. This oscillatory motion results in periodic vibrations characterized as squawk, shudder, or chatter. Stick-slip is most frequently observed at low sliding speeds and particularly when the coefficient of friction increases with decreasing sliding speed.
From a customer satisfaction view point, it is extremely important that the vehicle does not shudder at any point in its lifetime. OEM data show that shudder is more severe with new friction materials than after the materials are broken in. This means that for factory fill applications, the ATF must show good initial shudder performance before break-in as well as after break-in.
A need therefore exists for an effective way of overcoming the shudder problem associated with the continuous slip torque converter clutches for use in automatic transmissions, especially shudder which occurs with new friction materials before break-in. In fulfilling this need it is also important to ensure that the frictional characteristics needed in the automatic transmission fluid do not materially change with respect to time.
This invention overcomes the shudder problem by providing a friction modifier system that exhibits good anti-shudder performance both initially before break-in as well as after break-in. Moreover these performance advantages are achieved without material change in friction properties over time. Therefore, this invention now makes it possible for the original equipment manufacturers (OEMs) to make effective use of CSTCC designs in automatic transmissions in order to achieve the benefits made possible by such designs. And, as those skilled in the art can readily appreciate, there was no way by which the advantages of this invention could have been foreseen prior to the successful conduct of the experimental test work on this invention.